Op amps deliver 120dB EMI rejection ratio

National Semiconductor Corp. has introduced three operational amplifiers (op amps) with integrated EMI filters that maintain the accuracy of analogue systems by reducing the effects of RF interference. The LMV83x op amps are touted to deliver the industry's highest EMI rejection ratio (EMIRR) of 120dB, thereby eliminating EMI-induced errors.

The LMV831 single, LMV832 dual and LMV834 quad op amps feature CMOS inputs, a maximum input offset voltage of 1mV, a rail-to-rail output stage and an input common-mode voltage range that includes ground. These devices provide a common-mode rejection ratio (CMRR) and a power supply rejection ratio (PSRR) of 93dB. The LMV83x devices maintain stability for capacitive loads as large as 200pF and offer a temperature range of -40°C to 125°C.

The op amps also feature a 3MHz unity gain bandwidth while operating on only 240µA of supply current This yields a power-to-performance ratio of 80µA per MHz, which places these devices among National's PowerWise family of energy-efficient products. In addition, the LMV83x op amps reduce development time and board size by minimising the need for metal shielding, filters and extra components. They are well suited for use in phone accessories, medical instruments, precision weigh scales and other industrial electronic equipment that are sensitive to electromagnetic disturbances in noisy environments. Applications include filters/buffers, photodiode preamp and piezoelectric sensors.

For example, an injected RF signal in a weigh scale can result in as much as 1V of output offset, which would diminish a 10bit ADC's resolution (1,024 codes) to the equivalent of only 3bits, National said. Using a LMV83x op amp would only reduce resolution by 0.2bits.

To assist engineers with determining an op amp's EMI robustness, National developed the EMIRR specification, which quantifies how well various pins of the op amp reject RF interference. Evaluation boards are also available.

The LMV831 is offered in a 5-pin SC70 package, while the LMV832 is offered in an 8-pin MSOP package and the LMV834 is offered in a 14-pin TSSOP package. Pricing: The LMV831 is priced at Rs.23.57 (55 cents) each, the LMV832 at Rs.33.86 (79 cents) each, and the LMV834 at Rs.47.14 ($1.10) each, all in quantities of 1,000.

Samples of all three EMI-hardened op amps are available now and production quantities will be available in Q3 08.

What is an amplifier?
A device that uses a small amount of energy to control a larger amount. The relationship of the input to the output, expressed as a function of the input frequency, is called the transfer function of the amplifier, and the magnitude of the transfer function is called the gain.

What is EMI?
Electromagnetic interference or EMI is an electrical disturbance (interference or noise) in a system due to natural phenomena, low-frequency waves from electromechanical devices or high-frequency waves from ICs and other electronic devices.